Various devices are used in slipforming concrete into beams, curbs, hollow core slabs, and the like, in which concrete is extruded on the "long line" principle. These devices are utilized to form a variety of articles from plain concrete, reinforced concrete and prestressed (both pre-tensioned and post-tensioned) concrete. In the production of long slabs, concrete is precast with hollow cores on long beds or pallets, usually without transverse reinforcement. If transverse reinforcement is necessary when assembling floors or other slabs, to provide seismic "diaphragm" or for any other reason, then it is provided by either pouring a reinforced concrete topping over the slabs, when in place, or by welding or otherwise connecting projecting steel in the joints. Neither of these methods is very satisfactory and each is very expensive requiring large amounts of manual labor. It would be better to post-tension the slabs in place but the usual method of extrusion does not provide this ability.
Exemplary of modern extrusion techniques are the methods and apparatus described in U.S. Pat. Nos. 3,049,787, 3,159,897 and 3,284,687. These methods involve feeding concrete mix to one or more augers which force the mix into a molding section and out past a troweling section. The augers terminate in a mandrel which, by turning with the augers, shapes and forms hollow cores. The forming surfaces of the molding section are subjected to high amplitude vibration, at about 9-12,000 vibrations per minute, so as to compact and consolidate the concrete about the auger-driven mandrels. A disadvantage of such an "auger-mandrel" system is that the hollow cores are formed circular and must be located in the lines of flight of the augers.
The present invention provides slipforming apparatus which overcomes the foregoing deficiencies. The present invention utilizes the compacting force of rotating augers to consolidate "no-slump" concrete between form surfaces at a molding station and utilizes vibrations applied at the molding station. However, the vibrations are not utilized primarily to compress the cement mix, but rather to reduce friction at the form surfaces. Accordingly, whereas the prior art utilizes high amplitude, relatively low frequency vibrations, the present invention utilizes high frequency vibrations, in excess of 22,000 vibrations per minute, of relatively low amplitude. In another embodiment of the invention, friction is reduced by directing fluid, such as air or steam, under pressure between the form surfaces and the concrete mix. In contrast to prior methods, the present mode of operation enables the use of stationary mandrels which need not be located in the lines of flight of the augers and, in fact, are disposed axially lateral of such lines of flight. The result is that concrete articles can be formed with hollow cores of any desired configuration and placement.
Prestressed articles can be produced either for pre-tensioning or for post-tensioning. For post-tensioning, an elongate bar can be disposed in the extrusion path and the concrete mix consolidated thereabout so that as the bar is withdrawn, an elongate opening is defined through the extruded mix. The bar can be vibrated and, in this regard, consolidation of the concrete mix about the bar exerts a tensioning force which harmonically increases the vibrational frequency, thereby aiding in overcoming frictional forces. After the post-tensioning openings are formed, a prestressing strand can be threaded through the opening and tensioned. For this purpose, a novel air gun is provided which projects a leadwire through the opening.
For pre-tensioning, cables can be tensioned along the extrusion path and the concrete extruded thereabout. In this regard, prior sawing methods could not be used with prestressed "green" concrete since they require sawing through the prestressing strands in order to cut the article to a desired length. However, in accordance with the present invention, desired lengths of the article are obtained by introducing a plate into the extruded "green" mix transverse to the path of extrusion and vibrating the plate at a high frequency so as to reduce friction between the plate and the extruded mix. The vibrating plate is formed with cutouts for the prestressing cables so that the cables need not be cut at that point but can wait until the line has been completely formed and the concrete cured.
The present methods can be utilized to obtain transversely tensioned concrete articles. In this regard, a square slab of prestressed concrete can be extruded, the process stopped and the slab rotated so as to be positioned normal to the prior extrusion path. A second prestressed layer can then be deposited on the surface of the first slab to provide transverse reinforcement. Prior to extrusion of the second slab, a shear key is indented into the top surface of the first extruded slab, the second extrusion filling the indentations to securely and integrally form the article.
In still further embodiments, transverse reinforcement can be introduced into the concrete mix at the initial region of the molding station. In this regard, an overlap region can be formed between the auger flights and the form surfaces of the molding station, and U-shaped transverse ties (or other shapes where suitable) inserted at that point. Guide bars move the ties at extrusion speed through at least a consolidation region of the molding station.
The conveyors and molding station are movably mounted either with respect to a pallet or with respect to the ground so that as concrete is extruded, the apparatus moves in reaction therefrom. In the event that concrete is extruded directly on to a ground surface, such as when laying a median barrier strip on a highway, leveling mechanisms are utilized to maintain a uniform height and pitch.